The present invention relates to a method of and apparatus for treating refuse. More particularly this invention concerns a system for classifying refuse into several fractions.
For years refuse was usually burned or simply dumped outside. The incineration process not only requires the use of expensive equipment whose servicing and running is extremely expensive, but also usually due to the inadequate burning of the refuse creates an enormous amount of offensive solid ash and gases. The noxious vapors produced by incineration lead to rapid destruction of the equipment so that the process is quite expensive.
In order to achieve a better combustibility it has been suggested to coarsely comminute the refuse or trash before incinerating it. This makes it burn better and, when coupled with a subsequent drying operation, reduces the ash output considerably. It is also known to air-classify the refuse which has been coarsely comminuted by means of a hammer mill so as to separate from it stone, metal and glass particles before the burning operation. Thus all that is left to burn is the paper, wood and synthetic resin in the refuse. Not only does this considerably reduce the ash, but the heat produced by such combustion can often by exploited. With such a system, however, the treatment of the refuse prior to incineration nonetheless remains very expensive as the heat necessary to dry it and the energy necessary to operate the hammer mill represent a considerable operating cost.
It has been suggested to coarsely comminute the refuse by means of a hammer mill and then roughly classify it into a heavy fraction and a light fraction. The heavy fraction is then passed through a magnetic separator and the non-magnetic portion left is again air classified. The light fraction from the second classification and the originally obtained light fraction are then fed to another hammer mill and again classified so that the heavy organic particles and pieces in the refuse as well as the aluminum therein can be separated from the paper and synthetic-resin items in the trash. In such a system the trash or refuse is not dried so that a considerable saving in energy is obtained. Nonetheless the several milling operations as well as the several air-classifying operations do represent a considerable amount of equipment and, therefore, operating cost.
A further suggestion has been to first shake the refuse on a sieve so as to separate from the refuse ashes, sand and small particulate organic material. The remaining coarse refuse is then fed to a slicing-roller comminuter which cuts the refuse up into uniformly sized pieces whose maximum dimensions are between four and eight inches. The so-comminuted refuse is then air classified in a closed wind tunnel where the heavy synthetic-resin fraction is floated off and the ferromagnetic parts are drawn out by a large magnetic band, leaving only glass behind. Such a method has the advantage that it sorts the refuse into several distinct and readily recyclable fractions. Nonetheless the first comminution process is at least partially counter productive as it reduces many of the recyclable elements into smaller pieces that merely make the recycling operation more difficult. In any case this first coarse comminution step is nonetheless used in order to make the air classification as effective as possible. Even so a very strong wind current must be used to separate the various fractions. Thus it is necessary that this method be carried out in a closed chamber as the generation of dust and the like would surely be a pollution problem.
Another suggestion, seen in German patent 1,178,022, uses the method of placing the refuse on a rapidly moving horizontal transport band. This band is further reciprocated back and forth in a direction perpendicular to its carrying surface. This presorts the refuse. At its downstream end the refuse is thrown off with considerable force so that the particles with less weight quickly lose their energy and therefore fall close underneath the downstream end of the belt. However, the heavier particles have considerable inertia so that they are thrown relatively far from the sorting belt. Such classifying is helped by passing a current of air downwardly across the path of the refuse thrown from the belt.
Use of such a horizontal classifying band which is also vertically reciprocated nonetheless does not give altogether satisfactory results in that the trash is not surely separated into distinct classifications as the length of the trajectories followed by the items of trash varies as the belt reciprocates vertically. This gives the normally flat transport band the same disadvantage of a tossing roller in a classifying system, that is the items of trash should all have at least at the start the same trajectory so that a uniform classification is insured.